I think most of us use the GBs as a way for us all to get together for a common purpose, have some fun building for a particular subject, learn some history and share ideas. Since this is a fun hobby, it's not like having a work deadline, where missing it could negatively impact the business.
I know with the GBs I host, the dates are more of a guildline so that most of the participants are working on their projects at roughly the same time, which keeps the GB nice and active, and keeps the enthusiasm going. We have such a diverse group of people here, some people get started right away and only participate in a few GBs. Some of us get caught up in the fun and enthusiasm and want to participate in them all (such as myself... ), then end up scrambling at the end of each GB because we over comitted. And as we all know, real life has a tendency to get in the way or our fun at times.
Now if the GB was a competition, or a contest, then the deadline would be a very important part of it. Otherwise it's just like the Pirate's code, it's just a guildline.
(disclaimer, the rules of each GB are different. Be sure to read all rules and regulations before signing up. No purchase is necessary. No animals were used or harmed in any way for this event. All likenesses, names and personalities are fictionalized, and any similarities to any people, living or dead are coincidental. )